A battery charge management method and device operable to perform that method. The battery charge management method comprises the steps of determining charge delivered to or from a battery based on coulomb counting and updating an estimated battery state of charge based on the amount of delivered charge and measuring the battery current. Then comparing the updated estimated battery state of charge with a predetermined state of charge value of less than 100%, such that each predetermined state of charge value being associated with an expected battery current for that predetermined state of charge value. The method also includes modifying the updated estimated battery state of charge when the updated estimated state of charge is greater than the predetermined state of charge value and the measured battery current is greater than the expected battery current associated with the predetermined state of charge.
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15. A battery state of charge management device comprising:
means for measuring the battery current;
a coulomb counter for determining charge delivered to or from a battery based on coulomb counting and for updating an estimated battery state of charge based on the amount of delivered charge;
memory means in which is stored a selected recalibration point state of charge value of less than 100% and an expected battery current for that selected recalibration point state of charge; and
an analyzer for comparing the updated estimated battery state of charge with the selected recalibration point state of charge value and for modifying the updated estimated battery state of charge when the updated estimated state of charge is greater than the selected recalibration point state of charge value and the measured battery current is greater than the expected battery current associated with the selected recalibration point state of charge.
1. A battery state of charge management method comprising the steps of:
estimating a battery state of charge;
determining charge delivered to or from a battery based on coulomb counting and updating said estimated battery state of charge based on the amount of delivered charge;
measuring the battery current and comparing the updated estimated battery state of charge with a selected recalibration point state of charge having a value of less than 100%, the selected recalibration point state of charge having a value being associated with an expected battery current for that selected recalibration point state of charge value; and
modifying the updated estimated battery state of charge when the updated estimated state of charge is greater than the selected recalibration point state of charge value and the measured battery current is greater than the expected battery current associated with the selected recalibration point state of charge.
2. The battery charge management method of
3. The battery charge management method of
4. The battery charge management method of
5. The battery charge management method of
6. The battery charge management method of
7. The battery charge management method of
8. The battery charge management method of
9. The battery charge management method of
monitoring at least one characteristic of the battery representative of the battery's actual capacity; and
modifying the expected battery capacity used in determining charge delivered to or from the battery with respect to the monitored characteristic, wherein the monitored characteristic is the integral of depth of discharge measured over all charge cycles since manufacture of the battery.
10. The battery charge management method of
monitoring at least one characteristic of the battery representative of the battery's actual capacity; and
modifying the expected battery capacity used in determining charge delivered to or from the battery with respect to the monitored characteristic, wherein the characteristic of the battery representative of the battery's actual capacity is monitored at a predetermined point in the charging/discharging cycle of the battery; and
comparing the monitored characteristic with an expected value based on the expected battery capacity; and
modifying the expected battery capacity when the comparison identifies a difference between the monitored characteristic and the expected value.
11. The battery charge management method of
12. The battery charge management method of
13. The battery charge management method of
14. The battery charge management method of
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This application is a continuation of and claims priority to and the benefit of PCT Application No. PCT/GB2011/000187 filed on Feb. 11, 2011 which claims priority to and the benefit of Great Britain Application No. 1003507.9 filed on Mar. 3, 2010 and Great Britain Application No. 1002449.5 filed on Feb. 12, 2010.
The present invention relates to methods for managing the charging of batteries through the use of Coulomb counting. The present invention is applicable to, but not limited to, the charging of lead acid gel batteries for powering mobile radio base stations.
In order to charge a battery as rapidly as possible whilst simultaneously ensuring that the lifetime of the battery is prolonged, correct charging comprises a number of phases. During the bulk charging phase the charger is usually operating at its current limit and the voltage across the battery terminals increases as the battery is charged. At the end of the bulk charging phase the battery will hold a high proportionate charge, for example 80% of the maximum. When the bulk charging phase is completed the absorption charging phase begins, during this phase voltage is held constant at a comparatively high level which is achieved through regulation of the current. The transition between bulk charging and absorption charging can be determined by measuring the variation in the battery voltage. Overly prolonged absorption phase charging can be damaging to the battery, hence before damage can occur and typically before the battery is fully charged it is necessary to move to the float phase. During the float phase the voltage is reduced to a level where damage to the battery cannot occur, irrespective of how long the float phase lasts. Regulation of the current in both absorption and float phases also ensures that the battery does not overheat, thereby preventing any unwanted consequences for the longevity of the battery. At the end of a sufficiently long float phase the battery should be fully charged.
It is preferable for some battery types to continue charging until the battery is fully charged, so that the battery lifetime is prolonged. In the mobile radio base site power supply application it is desirable to identify the point at which the battery is fully charged, so that the generator can be switched off at the earliest opportunity in order to conserve fuel. Accurate determination of battery state of charge can enable the charging cycle to be completed in the absorption charging phase so that it is not necessary to go into a float phase. The state of charge value can also be used to trigger the activation of the generator to prevent the battery from being discharged too much, with the possible adverse consequences for battery longevity which would otherwise occur.
The state of charge of a battery can be determined by making measurements of battery voltage, however, this approach can be problematic for a number of reasons. One problem is that in some charging phases such as absorption there is little variation in voltage over a wide range of state of charge, which can lead to inaccurate estimation of state of charge. This can be problematic in situations, where it is necessary to configure the system to operate within a state of charge range for which the change in battery voltage is marginal, which might be the case for example when needing to prolong battery life. Another problem with voltage measurement based state of charge estimation is that battery voltage will be affected by battery temperature and the magnitude of the current which is being drawn, both of which would need to be compensated for.
An alternative method for determining battery state of charge is to monitor the charge that is flowing into and out of the battery in order to determine the state of charge of the battery at any particular moment, this method is called Coulomb counting. Using this technique it is possible to maintain an accurate estimate for the state of charge over all charging phases. The Coulomb counting approach also has a number of problems. One problem is that a method is needed for initializing the value to be used in the Coulomb count when the current state of charge of the battery is unknown, for example when a new battery is first connected to the charge management system. Another problem is that the Coulomb count may drift from the true value over time due, for example to measurement inaccuracy or internal discharge of the battery. A final problem is that the Coulomb count will give an inaccurate value for the percentage state of charge of the battery if the capacity of the battery has changed for any reason.
In a further aspect, in order to charge a battery as rapidly as possible whilst simultaneously ensuring that the lifetime of the battery is prolonged, correct charging comprises a number of phases. During the bulk charging phase the charger is usually operating at its current limit and the voltage across the battery terminals increases as the battery is charged. At the end of the bulk charging phase the battery will hold a high proportionate charge, for example 80% of the maximum. When the bulk charging phase is completed the absorption charging phase begins, during this phase voltage is held constant at a comparatively high level. The transition between bulk charging and absorption charging can be triggered when the measured battery voltage during bulk charging reaches a pre-determined threshold level. Overly prolonged absorption phase charging can be damaging to the battery, hence before damage can occur and typically before the battery is fully charged it is necessary to move to the float phase. During the float phase the voltage is reduced to a level where damage to the battery cannot occur, irrespective of how long the float phase lasts. Regulation of the current in the float phase may also be used to ensure that the battery does not overheat, thereby preventing any unwanted consequences for the longevity of the battery. At the end of a sufficiently long float phase the battery should be fully charged.
It is preferable for some battery types to continue charging until the battery is fully charged, so that the battery lifetime is prolonged. In the case of a diesel generator/battery hybrid power supply it is desirable to identify the point at which the battery approaches or reaches full charge, so that the generator can be switched off at the earliest opportunity in order to conserve fuel. Accurate determination of battery state of charge can enable the charging cycle to be completed in the absorption charging phase so that it is not necessary to go into a float phase. The state of charge value can also be used to trigger the activation of the generator to prevent the battery from being discharged too much, with the possible adverse consequences for battery longevity which would otherwise occur.
The state of charge of a battery can be determined by making measurements of battery voltage, however, this approach can be problematic for a number of reasons. One problem is that in some charging phases such as absorption the voltage level is regulated to a fixed level so that it cannot be used to determine state of charge. This can be problematic in situations where the system operates within a state of charge range corresponding to the absorption phase, for example when needing to prolong battery life or when availability of power from unreliable sources such as renewables is good. Another problem with voltage measurement based state of charge estimation is that battery voltage will be affected by battery temperature which would need to be compensated for.
An alternative method for determining battery state of charge is to monitor the charge that is flowing into and out of the battery in order to determine the state of charge of the battery at any particular moment; this method is called Coulomb counting. Using this technique it is possible to maintain an accurate estimate for the state of charge over all charging phases. The Coulomb counting approach also has a number of problems. One problem is that a method is needed for initialising the value to be used in the Coulomb count when the current state of charge of the battery is unknown, for example when a new battery is first connected to the charge management system. Another problem is that the Coulomb count may drift from the true value over time due, for example to measurement inaccuracy or internal discharge of the battery. A final problem is that the Coulomb count will give an inaccurate value for the percentage state of charge of the battery if the capacity of the battery has changed for any reason.
The present invention addresses some or all of the above problems that are encountered with Coulomb counting.
In a first aspect the present invention therefore provides a battery charge management method comprising the steps of determining charge delivered to or from a battery based on Coulomb counting and updating an estimated battery state of charge based on the amount of delivered charge; measuring the battery current; comparing the updated estimated battery state of charge with a predetermined state of charge value of less than 100%, each predetermined state of charge value being associated with an expected battery current for that predetermined state of charge value; and modifying the updated estimated battery state of charge when the updated estimated state of charge is greater than the predetermined state of charge value and the measured battery current is greater than the expected battery current associated with the predetermined state of charge.
Preferably, the battery charge management method further comprises the step of modifying the updated estimated state of charge to 100% when the measured battery current is less than a predetermined current value associated with the battery being fully charged.
Ideally, the predetermined state of charge value is determined at or near a point of greater than average change in the differential of battery state of charge expressed as a function of battery charging current. Also, when the battery's initial state of charge is unknown, an initial arbitrary value for the estimated battery state of charge is selected prior to the first update of the estimated state of charge.
In a specific embodiment, the step of modifying the updated estimated state of charge comprises the step of assigning the predetermined state of charge value to the updated estimated state of charge. Furthermore, charging of the battery may continue after the measured battery current equals or is less than the expected battery current associated with the predetermined state of charge value.
In a particular embodiment, the updated estimated state of charge is compared against a plurality of predetermined state of charge values, each predetermined state of charge value having an associated expect battery current, the updated estimated state of charge being modified when the updated estimated state of charge falls between two predetermined state of charge values but at least one of the two expected battery current values either side of the measured battery current is not associated with one of the two predetermined state of charge values either side of the updated estimated battery state of charge.
In a further aspect the present invention provides a battery management method comprising the steps of: determining charge delivered to or from a battery using Coulomb counting based upon an expected battery capacity; monitoring at least one characteristic of the battery representative of the battery's actual capacity; and modifying the expected battery capacity used in determining the charge delivered to or from the battery with respect to the monitored characteristic.
The monitored characteristic may be the number of charging cycles to which the battery has been subjected; the battery age; the integral of depth of discharge measured over all charge cycles since manufacture of the battery; the ambient temperature; and/or the difference between the battery temperature and the ambient temperature.
In a preferred embodiment the characteristic of the battery representative of the battery's actual capacity is monitored at a predetermined point in the charging/discharging cycle of the battery and the method further comprises the steps of: comparing the monitored characteristic with an expected value based on the expected battery capacity; and modifying the expected battery capacity when the comparison identifies a difference between the monitored characteristic and the expected value. When the comparison indicates the actual battery capacity to be lower than the expected battery capacity, the expected battery capacity may be reduced a small amount each charge cycle until substantially no difference is identified in the comparison step and when the comparison indicates the actual battery capacity to be higher than the expected battery capacity, the expected battery capacity may be increased by a small amount each charge cycle until substantially no difference is identified in the comparison step.
The monitored battery characteristic may be the voltage level of the battery at the end of a discharge period or the monitored battery characteristic may be the battery charging current at a selected state of charge. In the latter case, the curve of battery charging current with respect to battery state of charge may be compared with a set of pre-determined mappings.
In a further preferred embodiment, the battery may comprise a string of battery cells and the expected battery capacity may be modified when it is determined that one or more battery cells have ceased to function. In this embodiment the method may further comprise the step of comparing the voltage measured across half the cells in the battery string with the voltage measured across the other half of the battery cells in the battery string and determining whether one or more battery cells have ceased to function on the basis of a difference between the measured voltages. This enables the number of failed battery cells to be estimated according to the magnitude of the difference between the two measured voltages and when it is determined a modification of the expected battery capacity is required, the expected battery capacity may be modified by an amount determined with respect to a look-up table of battery capacity versus number of failed cells.
It will, of course, be apparent that the battery management method described above may be combined with the battery charge management method also described above in which case the predetermined state of charge value may be changed when the expected battery capacity is modified.
In a further aspect the present invention provides a battery charge management device comprising: means for measuring the battery current; a Coulomb counter for determining charge delivered to or from a battery based on Coulomb counting and for updating an estimated battery state of charge based on the amount of delivered charge; memory means in which is stored a predetermined state of charge value of less than 100% and an expected battery current for that predetermined state of charge; an analyser for comparing the updated estimated battery state of charge with the predetermined state of charge value and for modifying the updated estimated battery state of charge when the updated estimated state of charge is greater than the predetermined state of charge value and the measured battery current is greater than the expected battery current associated with the predetermined state of charge.
In a still further aspect the present invention provides a battery management device comprising: a Coulomb counter for determining charge delivered to or from a battery using Coulomb counting based upon an expected battery capacity; a controller for monitoring at least one characteristic of the battery representative of the battery's actual capacity; and for modifying the expected battery capacity used in determining the charge delivered to or from the battery with respect to the monitored characteristic.
The battery management device may further comprise a temperature monitor for monitoring one or both of ambient temperature and battery temperature.
Alternatively, the controller may be adapted to monitor the at least one characteristic of the battery at a predetermined point in the charging/discharging cycle of the battery and may be adapted to compare the monitored characteristic with an expected value based on the expected battery capacity so as to modify the expected battery capacity when the comparison identifies a difference between the monitored characteristic and the expected value.
Thus, according to the present invention a battery charge management method and apparatus based on Coulomb counting are provided which correct for any divergence between an estimated state of charge and an actual state of charge, by modifying the estimated battery state of charge value on the occasions when the measured battery charging current is inconsistent with the expected battery charging current for the given estimated state of charge. Also according to the present invention a battery management method and apparatus are provided which enable the expected maximum battery capacity used in Coulomb counting to be adjusted either alone or in combination with adjustment of the expected state of charge.
Also disclosed herein are additional solutions that overcome the problems in the prior art that are encountered with Coulomb counting.
In another aspect the present invention therefore provides a battery charge management method comprising the steps of: determining charge delivered to or from a battery based on Coulomb counting and updating an estimated battery state of charge based on the amount of delivered charge; measuring the battery current; comparing the updated estimated battery state of charge with a predetermined state of charge value of less than 100%, each predetermined state of charge value being associated with an expected battery current for that predetermined state of charge value; and modifying the updated estimated battery state of charge either: i) when the updated estimated state of charge is greater than the predetermined state of charge value or equal to the predetermined state of charge and the measured battery current is greater than the expected battery current associated with the predetermined state of charge or equal to the expected battery current associated with the predetermined state of charge, or ii) when the estimated state of charge is not equal to the pre-determined state of charge but the measured battery current is substantially equal to the expected battery current associated with the predetermined state of charge.
Preferably, the battery charge management method further comprises the step of modifying the updated estimated state of charge to an end point state of charge value when the measured battery current is at least less than a predetermined current value at which further charging of the battery is halted. In a preferred embodiment the end point state of charge value is around 97%.
When the battery's initial state of charge is unknown, an initial arbitrary value for the estimated battery state of charge may be selected prior to the first update of the estimated state of charge.
In a specific embodiment, the step of modifying the updated estimated state of charge comprises the step of assigning the predetermined state of charge value to the updated estimated state of charge. Furthermore, charging of the battery may continue after the measured battery current equals or is less than the expected battery current associated with the predetermined state of charge value.
In a particular embodiment, the updated estimated state of charge is compared against a plurality of predetermined state of charge values, each predetermined state of charge value having an associated expect battery current, the updated estimated state of charge being modified when the updated estimated state of charge falls between two predetermined state of charge values but at least one of the two expected battery current values either side of the measured battery current is not associated with one of the two predetermined state of charge values either side of the updated estimated battery state of charge.
Moreover, preferably the estimated battery state of charge is only modified during a charging phase in which battery voltage is regulated to the absorption voltage, more preferably during a battery charging phase when the available current to charge the battery is greater than the current which can be drawn by the battery. When the battery is charged at least in part using current supplied by a wind turbine, the available current may be determined with respect to a measurement of wind speed and/or when the battery is charge at least in part using current supplied by a photo-voltaic panel, the available current may be determined with respect to a measurement of solar insolation.
In a further aspect the present invention provides a battery management method comprising the steps of: determining charge delivered to or from a battery using Coulomb counting wherein the maximum amount of charge that can be delivered to or from the battery is based upon an expected battery capacity; monitoring at least one characteristic of the battery representative of the battery's actual capacity; and modifying the expected battery capacity in accordance with the monitored characteristic.
The monitored characteristic may be the number of charging cycles to which the battery has been subjected; the battery age; the integral of depth of discharge measured over all charge cycles since manufacture of the battery; the ambient temperature; and/or the difference between the battery temperature and the ambient temperature.
In a preferred embodiment the characteristic of the battery representative of the battery's actual capacity is monitored at a predetermined point in the charging/discharging cycle of the battery and the method further comprises the steps of: comparing the monitored characteristic with an expected value based on the expected battery capacity; and modifying the expected battery capacity when the comparison identifies a difference between the monitored characteristic and the expected value. When the comparison indicates the actual battery capacity to be lower than the expected battery capacity, the expected battery capacity may be reduced a small amount each charge cycle until substantially no difference is identified in the comparison step and when the comparison indicates the actual battery capacity to be higher than the expected battery capacity, the expected battery capacity may be increased by a small amount each charge cycle until substantially no difference is identified in the comparison step.
The monitored battery characteristic may be the voltage level of the battery at the end of a discharge period or the monitored battery characteristic may be the battery charging current at a selected state of charge. In the latter case, the curve of battery charging current with respect to battery state of charge may be compared with a set of pre-determined mappings.
In a further preferred embodiment, the battery may comprise a string of battery cells and the expected battery capacity may be modified when it is determined that one or more battery cells have ceased to function. In this embodiment the method may further comprise the step of comparing the voltage measured across half the cells in the battery string with the voltage measured across the other half of the battery cells in the battery string and determining whether one or more battery cells have ceased to function on the basis of a difference between the measured voltages. This enables the number of failed battery cells to be estimated according to the magnitude of the difference between the two measured voltages and when it is determined a modification of the expected battery capacity is required, the expected battery capacity may be modified by an amount determined with respect to a look-up table of battery capacity versus number of failed cells.
It will, of course, be apparent that the battery management method described above may be combined with the battery charge management method also described above in which case the predetermined state of charge value may be changed when the expected battery capacity is modified.
In a further aspect the present invention provides a battery charge management device comprising: means for measuring the battery current; a Coulomb counter for determining charge delivered to or from a battery based on Coulomb counting and for updating an estimated battery state of charge based on the amount of delivered charge; memory means in which is stored a predetermined state of charge value of less than 100% and an expected battery current associated with the predetermined state of charge; an analyser for comparing the updated estimated battery state of charge with the predetermined state of charge value and for modifying the updated estimated battery state of charge either: i) when the updated estimated state of charge is greater than the predetermined state of charge value or equal to the predetermined state of charge value and the measured battery current is greater than the expected battery current associated with the predetermined state of charge or equal to the expected battery current associated with the predetermined state of charge, or ii) when the estimated state of charge is not equal to the pre-determined state of charge but the measured battery current is substantially equal to the expected battery current associated with the predetermined state of charge.
In a still further aspect the present invention provides a battery management device comprising: a Coulomb counter for determining charge delivered to or from a battery using Coulomb counting wherein the maximum amount of charge that can be delivered to or from the battery is based upon an expected battery capacity; a controller for monitoring at least one characteristic of the battery representative of the battery's actual capacity; and for modifying the expected battery capacity in accordance with the monitored characteristic.
The battery management device may further comprise a temperature monitor for monitoring one or both of ambient temperature and battery temperature.
Alternatively, the controller may be adapted to monitor the at least one characteristic of the battery at a predetermined point in the charging/discharging cycle of the battery and may be adapted to compare the monitored characteristic with an expected value based on the expected battery capacity so as to modify the expected battery capacity when the comparison identifies a difference between the monitored characteristic and the expected value.
Thus, according to the present invention a battery charge management method and apparatus based on Coulomb counting are provided which correct for any divergence between an estimated state of charge and an actual state of charge, by modifying the estimated battery state of charge value on the occasions when the measured battery charging current is inconsistent with the expected battery charging current for the given estimated state of charge. Also according to the present invention a battery management method and apparatus are provided which enable the expected maximum battery capacity used in Coulomb counting to be adjusted either alone or in combination with adjustment of the expected state of charge.
Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.
Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
When the system is initially commissioned or when a system reset is performed, such that current state of charge data is either unavailable or has been lost then the estimate for the percentage state of charge, ‘SoC(n)’ of the battery which is estimated by the algorithm is set to an arbitrary value ‘Initial SoC’, for example 90%. Then once every second the Coulomb count is updated to account for the charge that has flowed into or out of the battery during the one second period. In order to update the Coulomb count a measurement of the ‘battery current’ flowing into or out of the battery is made. If the current is flowing into the battery then ‘battery current’ takes a positive value, if the current is flowing out of the battery and the battery is discharging into the load then ‘battery current’ takes a negative value. Since 1 Ampere is equivalent to a charge flow of 1 Coulomb per second, the change in ‘SoC’ is given by ‘battery current’ multiplied by 100 divided by the maximum charge carrying capacity of the battery (measured in Coulombs). In addition, during each one second loop a number of checks are performed to see whether the assumed ‘SoC’ needs to be corrected. Any correction in the assumed ‘SoC’ value is performed by comparing the estimated SoC with one or more pre-selected expected SoC values. These corrections can be performed when measured charging currents reach one or more pre-selected current values: either when measured current matches the current at the knee point of the curve of battery state of charge versus battery current, see
In
In
In
It may be noted that when the system is initially commissioned the battery charge management algorithm will not gain an accurate value for battery state of charge until the battery is approaching full charge. This is acceptable in applications where, in the first charge cycle following commissioning the main purpose of the Coulomb counting algorithm will be to determine the point at which the battery reaches full charge so that the generator can be switched off. Note that the change in current and voltage regulation required as the charging moves from the bulk charging phase to the absorption charging phase can be triggered by alternative methods such as through monitoring of the battery voltage level.
An alternative way of correcting for the possible drift between computed Coulomb count and actual Coulomb count is to periodically perform an extended run in the float phase at the end of which there can be a high level of confidence that the battery will have reached full charge after which the assumed value for state of charge can be set accordingly at 100%. However, this is more costly in energy consumption terms since charging current needs to be supplied for longer than is necessary.
The capacity of a battery will decrease over time as it ages and/or as battery cells cease to be effective. However, the battery charging algorithm requires an accurate value for maximum battery capacity (measured in Ah). This is because if the algorithm uses a value for battery capacity which is higher than what it actually is then the depth of discharge will be more than is desirable, since the Coulomb counting algorithm will allow more charge to flow out of the battery than is desirable, which will reduce battery longevity. Hence there is a need to determine the changing capacity of the battery, ‘max battery capacity’ as it ages and to then apply the new value in the battery charge management algorithm (
The curve of battery state of charge versus charging current will change in the event that a cell fails, since the string with the failed cell will become more resistive.
Finally, another method for detecting changes in the values which should be applied in the charge management algorithm would be to monitor charging current following an extended run in the float phase, following which it would be known with a high degree of probability that the battery is fully charged. The charging current measurement could then be used to perform a table look up of the other values, for example knee point SoC and knee point charging current that are required by the Coulomb counting algorithm. Because there is a cost in terms of diesel consumption in charging the battery during the float phase, such a re-calibration of algorithmic values would be done relatively infrequently, for example typically less than once a week.
Changes to the apparatus and methods described in detail above are envisaged without departing from the scope of the invention as set out in the accompanying claims. In particular, in
Also disclosed herein are methods for managing the charging of batteries through the use of Coulomb counting. This is applicable to, but not limited to, the charging of lead acid gel batteries for powering mobile radio base stations.
When the system is initially commissioned or when a system reset is performed, such that current state of charge data is either unavailable or has been lost then the estimate for the percentage state of charge, ‘SoC(n)’ of the battery which is estimated by the algorithm is set to an arbitrary value ‘Initial SoC’, for example 90%. Then once every second the Coulomb count is updated to account for the charge that has flowed into or out of the battery during the one second period. In order to update the Coulomb count a measurement of the ‘battery current’ flowing into or out of the battery is made. If the current is flowing into the battery then ‘battery current’ takes a positive value, if the current is flowing out of the battery and the battery is discharging into the load then ‘battery current’ takes a negative value. Since 1 Ampere is equivalent to a charge flow of 1 Coulomb per second, the change in ‘SoC’ is given by ‘battery current’ multiplied by 100 divided by the maximum charge carrying capacity of the battery (measured in Coulombs). In addition, during each one second loop a number of checks are performed to see whether the estimated ‘SoC’ needs to be corrected. Any correction in the estimated ‘SoC’ value is performed by comparing the estimated SoC with one or more pre-selected expected SoC values. These corrections can be performed when measured charging currents reach one or more pre-selected current values: either when measured current matches the current at the re-calibration point of the curve of battery state of charge versus battery current, see
In
In
In
It may be noted, with respect to the algorithm of
Note that the system will only be in the absorption phase with voltage being regulated if the current that can be supplied by the energy sources is greater than the current which is being drawn by the battery. For this reason, in the absorption phase the battery charging current is determined purely by the state of charge of the battery and is not dependent on the current supply capabilities of the energy sources.
Note that for a simple system consisting of just a diesel generator based charger and a battery supplying power to a mobile radio base station there will typically be one charge and one discharge per day. The main requirement during charging is to determine when to switch off the generator. Because it is known with some certainty that the generator will be able to supply power through both bulk and absorption phases, it would be possible to determine when the generator should be switched off simply by monitoring charging current and switching off the generator when the charging current is low enough. The main requirement during discharging is to determine when to switch the generator on again. During discharge there is no current to measure and the state of charge as given by the Coulomb counter is required to determine when to switch the generator off. However for other more complex power supply configurations it is essential to keep track of battery state of charge during charging as well as discharging. An example of a more complex scenario would be one in which there is a source of power which is less reliable and less controllable than a generator, for example a scenario in which there is a renewable energy component or a grid connection which only supplies power sporadically during the day. In these scenarios there may be multiple small scale charge and discharge cycles occurring throughout the day during which the Coulomb count during both charging and discharging should be maintained. Tracking of the SoC level is needed so that the charge management algorithm can determine when and if the generator should be switched on. Since charging current may be a function of both the maximum current which can be supplied by the energy source(s) as well as the charge acceptance of the battery, where a variable output power renewable energy source is charging the battery, it won't necessarily be possible during charging to track the state of charge by monitoring charging current alone.
In one embodiment of the invention a check is performed to confirm that the current which can be supplied by the energy sources exceeds the current which is being drawn by the battery, and only when this is the case is a re-calibration based on measured charging current allowed to occur. In order to determine the maximum charging current which can be supplied by a given wind turbine configuration at any given instant a measurement of wind speed may be made. In order to determine the maximum charging current which can be supplied by a given photo-voltaic panel configuration at any given instant a measurement of solar insolation may be made.
For power supply scenarios which include only less reliable energy sources such as renewables or low availability grid connections, there are additional benefits in correcting erroneous SoC at the earliest possible opportunity, i.e. when SoC is still relatively low or equivalently when charging current is still high as will occur shortly after the system has gone into the absorption phase. This is beneficial because in such systems the point at which discharge will occur cannot be controlled, and for this reason SoC re-calibration should be carried out at the earliest possible opportunity.
There are also benefits in correcting erroneous SoC at the earliest possible opportunity in power supply scenarios which include a combination of less reliable energy sources such as renewables and a diesel generator. In such scenarios it may be desirable, for example to switch the generator off at state of charge levels well below full charge if weather forecasts indicate that there may be sufficient wind or solar energy coming. In such scenarios the battery could be charged from say 90% to 97% using renewables and without running the generator. So the generator could be switched off at 90%, thereby saving fuel. If re-calibration of SoC is achievable at a value less than 90% then re-calibration of SoC would have been achieved even if the projected wind or solar energy did not in fact materialise.
An alternative way of correcting for the possible drift between computed Coulomb count and actual Coulomb count is to periodically perform an extended run in the float phase at the end of which there can be a high level of confidence that the battery will have reached full charge after which the assumed value for state of charge can be set accordingly at 100%. However, this is more costly in energy consumption terms since charging current needs to be supplied for longer than is necessary.
The capacity of a battery will decrease over time as it ages and/or as battery cells cease to be effective. However, the battery charging algorithm requires an accurate value for maximum battery capacity (measured in Ah). This is because if the algorithm uses a value for battery capacity which is higher than what it actually is then the depth of discharge will be more than is desirable, since the Coulomb counting algorithm will allow more charge to flow out of the battery than is desirable, which will reduce battery longevity. Hence there is a need to determine the changing capacity of the battery, ‘max battery capacity’ as it ages and to then apply the new value in the battery charge management algorithm.
The curve of battery state of charge versus charging current will change in the event that a cell fails, since the string with the failed cell will become more resistive.
Changes to the apparatus and methods described in detail above are envisaged without departing from the scope of the invention as set out in the accompanying claims. In particular, in
Particular aspects and embodiments are described further in the following numbered paragraphs:
It will be appreciated that the present invention is applicable to, but not limited to, the charging of lead acid batteries.
A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods.
The functions of the various elements shown in the Figures, including any functional blocks labelled as “processors” or “logic”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” or “logic” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the Figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.
It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.
The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
Barrett, Stephen, Johnson, Graham Ronald, Bishop, Peter William Dale
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